1. Field of the Invention
The present invention relates generally to the field of optical imaging and more particularly to catadioptric optical systems used for microscopic imaging, inspection, and lithography applications.
2. Description of the Related Art
Many optical and electronic systems exist to inspect surface features for defects such as those on a partially fabricated integrated circuit or a photomask. Defects may take the form of particles randomly localized on the surface, scratches, process variations, and so forth. Such techniques and devices are well known in the art and are embodied in various commercial products such as many of those available from KLA-Tencor Corporation of San Jose, Calif., the assignee of the present application.
Specialized optical systems are required to enable the imaging and inspection of surface defects, such as those found on semiconductor wafers and photomasks. Two prior inventions describe high numerical aperture (NA) catadioptric systems that can support this type of imaging. These inventions are U.S. Pat. No. 5,717,518 to Shafer et al., as shown in FIG. 1, and U.S. Pat. No. 6,064,517, also to Shafer et al.
U.S. Pat. No. 5,717,518 to Shafer et al. describes an apparatus capable of high NA, ultra broadband UV imaging. The '518 patent describes a 0.9 NA system that can be used for broadband bright field and multiple wavelength dark-field imaging. It has ultra broad band chromatic correction using an achromatized field lens. This system can employ an achromatized field lens group to correct for secondary and higher order lateral color. This type of design has several limitations including a limited working distance, central obscuration, internal pupil, and some relatively tight manufacturing tolerances. The tight manufacturing tolerances mainly come from the balance of the large spherical aberration generated from the catadioptric group.
U.S. patent application Ser. No. 09/349,036 filed on Jul. 7, 1999 to Shafer et al., as shown in FIG. 2, describes a high NA optical apparatus that has several advantages including a long-working distance, zero central obscuration, external pupil, and a relatively loose manufacturing tolerance. The system presented in the '036 application is ideally suited for use in long-working distance imaging applications, such as reticle inspection and lithography. This design is highly applicable for use in both 193 nm and 157 nm generation inspection and lithography applications. The design shown has a high degree of chromatic correction using a single glass material. Further chromatic correction is possible using two glass materials. This design has a long overall optical path, which in certain circumstances can adversely affect mechanical stability and manufacturing costs. Also, the optical system has an unusual optical axis near perpendicular that can offer manufacturing challenges.
Other specialized catadioptric optical systems have been developed for use in semiconductor lithography. These systems are designed to image a photomask at a reduced magnification onto a resist coated wafer. U.S. Pat. No. 5,052,763 to Singh et al. discloses a catadioptric optical system capable of high NA imaging. The '763 design creates a substantially flat image field over the large areas required for semiconductor lithography by having an input optical system with a curved field, a catadioptric relay system, and an output optical system to correct for the field curvature and some monochromatic aberrations. This design has several limitations including a limited working distance, internal pupil, narrow bandwidth, an internal beamsplitter, and tight manufacturing tolerances.
It is therefore an object of the present invention to provide an apparatus that has a long working distance between the optical system and the surface being inspected, a high numerical aperture, and small size.
It is also an object of the present invention to provide an apparatus that has a high degree of chromatic correction using a single glass material, where further chromatic correction can be achieved using at least one additional glass material, thereby making the apparatus suited for use at wavelengths in the deep UV and vacuum UV ranges.
It is still another object of the present invention to provide an apparatus with an external pupil plane to support apertureing and Fourier filtering.
It is another object of the present invention to provide an apparatus having relatively loose tolerances enabling manufacture for a reasonable cost.
It is another object of the present invention to provide an apparatus that has excellent image quality and a high degree of chromatic correction without the requirement to use aspherics, diffractive optics, beam splitters, or turning mirrors.
It is a further object of the present invention to provide an apparatus that is suited to support microscopic imaging and inspection applications at wavelengths in the UV to VUV spectral range.
It is another object of the present invention to provide an apparatus to support both broadband bright field and laser dark field imaging and inspection.